High glide refrigerant blends such as HFC R407C, HCFC R409A and other zeotropic refrigerant blends (to a lesser extent HFC R410A and HFC R507) fractionate during transfers of the blends from one container (e.g., tank or cylinder) to another causing the composition of the blend to change. This change in composition can make the product off specification, change it's performance and/or make the material hazardous.
With the phase out of CFCs (implicated ozone depleting materials), the refrigeration and air condition industry has had to use substitute blends that are optimized based on many different properties. Ideally, replacement refrigerant compositions should have the same thermodynamic properties as the composition being replaced, as well as chemical stability, low toxicity, non-flammability and efficiency-in-use. Unfortunately, single component replacement refrigerants are often unable to provide all of the desired properties. In order to match the properties of the refrigerants being replaced, blends of environmentally acceptable refrigerants have been developed to achieve the best possible performance, capacity, efficiency and safety, as well as minimal cost. Blends of liquids, however, can fractionate.
A liquid heated above its boiling point changes phases to a vapor, and a vapor cooled below its condensation point changes phases to a liquid. For pure, single component fluids the boiling point and condensation point temperatures at a given pressure are the same, and the composition of such a fluid is the same in its vapor and liquid states. Fluids can also change state due to a change of pressure. When the pressure on a liquid is lowered below the vaporization pressure it becomes a vapor, and when the pressure is increased above its condensation pressure, it becomes a liquid. For a pure, single component fluid the vaporization and condensation point pressures at a given temperature are the same, and the composition of such a fluid remains constant.
For blends of fluids having different thermodynamic properties, however, such as refrigerant blends, the relationship between vaporization and condensation is more complex. In such fluid mixtures, boiling or condensation may occur over a range of temperatures rather than at a single fixed point. For example, for non-azeotropic blends (also referred to as zeotropic blends) as the temperature of such a fluid mixture in liquid state is raised, the lower boiling-point components boil off preferentially. The point at which the liquid first begins to vaporize is referred to as the bubble point, i.e. the point at which bubbles first form. The bubble point can be expressed as the temperature above which a constant pressure liquid begins to vaporize, or it can be expressed as the pressure below which a constant temperature liquid begins to vaporize, also referred to as the bubble point pressure. Conversely, for such a blend in vapor state, as the temperature of the vapor is lowered, the highest condensation temperature components begin to condense first. The point at which vapor first begins to condense is referred to as the dew point. The dew point can be expressed as the temperature below which a constant pressure vapor begins to vaporize, or it can be expressed as the pressure above which a constant temperature vapor begins to condense, also referred to as the dew point pressure. Thus, a fluid blend begins to vaporize at its bubble point, and completes the vaporization at its dew point, and vice versa. The range between the bubble and dew points is often referred to as the “glide.”
Because of the different boiling points of the components of such blends, the fluids tend to segregate or fractionate during boiling. That is, as the temperature increases, the lower boiling point components vaporize preferentially. This results in the vapor having a higher concentration of the lower boiling components than the liquid, and a lower concentration of the higher boiling components. This effect is referred to as segregation or fractionation. As a result, when such a fluid blend is stored in a closed container in which there is a vapor space above a quantity of liquid, the composition of the vapor is different from that of the liquid. When such blends are withdrawn from the container in which they are stored, fractionation of the liquid remaining in the container can take place, with accompanying changes in composition of the remaining liquid. Composition changes of the mixture can be quite significant, and even relatively small composition changes cannot be tolerated in certain circumstances. Such changes can cause a refrigerant to have a composition outside of specified limits, to have different performance properties or even to become hazardous, such as by becoming flammable.
The problem of fractionation is a particular problem for high-glide refrigerants because of the greater tendency of the low and high boiling point components to segregate. On the other hand, pure single component fluids have zero glide. The composition of the initial vapor is the same as that of the final vapor as the liquid boils off. Therefore, they do not experience the compositional changes of high-glide fluid blends during vaporization.
It is a standard practice to make up blends in large tanks which then are used to fill cylinders for sale and use. As discussed above, in transferring from the bulk or source tank, the composition of the liquid remaining in the source tank can change. The vapor above the liquefied blend has a different composition then the liquid. This can lead to a change in composition as the liquid is removed from the container such that it the remaining liquid occupies a different volume. This shift in composition is undesirable since it can lead to changes in performance, efficiency and safety of the blend.
ASHRAE (American Society of Heating, Refrigeration and Air Conditioning Engineers) and ARI (Air-Conditioning and Refrigeration Institute) have recognized these problems and have examined the effect of the shift in composition. A recognized problem is that in normal transfers this fractionation can change the refrigerant blend composition sufficiently such that the blend will no longer be within the tolerances originally set. A means of transferring that can avoid this undesirable effect is needed.
One means of dealing with this problem has been to use one-use packages. Here, a cylinder contains the exact quantity of material needed for a given application, the liquid material stored within being completely used in that one use. This is not practical in the air conditioning and refrigeration industry due to the wide variety of equipment and charge sizes required. The number of different size package would be too large to stock and manage economically.
Another known means to prevent fractionation is to have only a single phase present in the cylinder of refrigerant. Using only liquid in the cylinder is not practical due to temperature changes and during use the liquid filled condition is not maintained. If only vapor is used, the tank contains much less material or must be very large. Therefore, this approach is rarely practical.
A common method has been to remove only liquid from the container. This is not ideal. This causes far less fractionation then removing vapor but the composition still does shift and in some situation by more than can be tolerated in the refrigeration and air-conditioning industry. An improvement on this idea was to mix some vapor with the liquid as it was removed, using a perforated dip tube as described in U.S. Pat. No. 3,656,657. This method has not been widely used, probably due to the flow rate dependency.
The use of a bladder has been used in packages in the past and has the potential to solve this problem. The concept is used to prevent fractionation of the refrigerant blend during dispensing by preventing a vapor space from forming, see e.g., U.S. Pat. No. 6,234,352.
Accordingly, it is an object of the invention to provide a method and apparatus the permits dispensing of a refrigerant blend with minimal fractionation.
Other objects and advantages of the invention will become apparent from the following description of the invention.